Xenoestrogens (XE), compounds that mimic the action of estrogen, are present in the environment and represent a public health concern due to potentially disruptive effects on physiological processes regulated by estrogen including reproduction, fetal development, and the development of cancers of reproductive tissues. There are numerous examples of reproductive anomalies in wildlife born in ecosystems contaminated with XE, highlighting the potential consequences of human exposure. The inability to reliably predict estrogenicity based on structural analysis necessitates the development of functional screening systems to safeguard human health. The transactivation assay, in which cells are cotransfected with the cDNA for ERalpha and a reporter gene containing the Vitellogenin A2 (Vit) estrogen response element (ERE) has been utilized to assess ERalpha-mediated transcriptional activity of XEs. However, reports of gene-specific transcriptional activity exhibited by XEs, as well as other estrogen analogs, suggest that estrogenicity is not merely an inherent property of a ligand, but also a function of the specific gene and cellular context. Variability in ERE sequence and/or cellular complements of coactivators/corepressors may contribute to differential patterns of gene activation. Therefore, to provide a more complete assessment of the biological activity of XEs, transactivation assays will be carried out utilizing reporter plasmids containing ERE sequences derived from the pS2 and lactoferrin genes, in addition to the Vit ERE. Transcription will be assessed in yeast and RL95-2 human endometrial carcinoma cell lines transfected with the mouse ERalpha and reporter constructs containing the aforementioned EREs. Established reporters of the cell- and gene-specific transcriptional activity of other estrogen analogs demonstrated the importance of this experimental approach in establishing an accurate profile of the estrogenic potency of XEs. The findings will generate substantive information regarding the estrogenicity of these compounds, the effect of ERE sequence and cell type on transcriptional efficacy, and the utility of the in vitro transactivation assay as a functional screening system for XEs.